Ran out of back ink the other day while printing labels from my inkjet. My blacks started coming out grey.

Then I remembered that some manufacturers used to ship printers without the black cartridge. If you overlay cyan magenta and yellow inks you get black of course. So I pulled out the empty black ink cart.

My HP printer showed an error "cartridge missing", and appeared stuck in this state, until I sent a print, at which time the printer came to life and printed a nice dark label for me.